1. Field of the Invention
The present invention relates to a member for a semiconductor structure, and more particularly, it relates to a substrate for a semiconductor device or structure, which must be of high thermal conductivity to be mounted with a semiconductor device of high calorific power such as a high-power transistor or a laser diode.
2. Background Information
A member for a semiconductor structure to be mounted with a semiconductor device is generally formed by an insulating member and a radiating member joined to the insulating member. For example, such a member for a semiconductor structure is formed by an insulating substrate to be provided thereon with a semiconductor device and a radiating substrate joined to the back surface of the insulating substrate by soldering with silver solder or the like. In this case, generally the substrate must have the following qualities, a high electric insulating ability for insulating the semiconductor device, high mechanical strength, and a high thermal conductivity for dissipating heat generated by the semiconductor device. The radiating or heat removing substrate must have a high thermal conductivity similarly to the insulating substrate, while its thermal expansion coefficient must approximate those of the materials forming a semiconductor substrate, the insulating substrate and the like.
In general, alumina (Al.sub.2 O.sub.3) is selected as a material satisfying the aforementioned properties for forming the insulating substrate employed in such a member for a semiconductor structure. However, although alumina is excellent in electric insulability and mechanical strength, its heat dissipation property is inferior due to a small thermal conductivity of 17 Wm.sup.31 1 K.sup.31 1. Thus, it is improper to mount a field-effect transistor (FET) of high calorific power, for example, on an alumina substrate. In order to mount a semiconductor device of high calorific power, another type of insulating substrate is made of beryllia (BeO) having a high thermal conductivity of 260 Wm.sup.-1 K.sup.-1. However, beryllia is toxic and hence it is troublesome to assure the necessary safety measures when using such an insulating substrate.
The radiating substrate or rather element is generally made of material satisfying the aforementioned properties, which material is selected from metal materials such as various types of copper alloys, copper-tungsten alloys and copper-molybdenum alloys. For example, Japanese Patent Laying-Open Gazette No. 21032/1984 discloses a substrate of high thermal conductivity for carrying a semiconductor device, the material of which is prepared by mixing 2 to 30 percent by weight of copper into tungsten or molybdenum. This substrate is employed as a radiating substrate which is suitably joined to an alumina substrate having an inferior heat dissipation property. However, the difference between the thermal expansion coefficients of the radiating substrate and the alumina substrate is relatively small. Thus, this prior art example has an insufficient heat dissipation property, which is required of an entire substrate for carrying a semiconductor device.
In recent years, nontoxic aluminum nitride (AlN) has generated a great interest as a material for such an insulating substrate for mounting a semiconductor device of high calorific power because of its high thermal conductivity of about 200 Wm.sup.-1 K.sup.-1, which value is substantially equal to that of beryllia, as well as its electric insulation ability and mechanical strength which are equivalent to those of alumina.
However, when an aluminum nitride substrate provided with a metallized layer, is soldered by a soldering metal such as a gold solder or a silver solder, for example, to a generally employed radiating or heat sink element of a copper-tungsten alloy or copper-molybdenum alloy containing 10 to 25 percent by weight of copper, the aluminum nitride substrate may crack or the radiating element of the copper-tungsten alloy or the copper-molybdenum alloy may warp.
Cracking or warping results from thermal stress caused by a difference in the thermal expansion coefficients between the copper-tungsten alloy or the copper-molybdenum alloy and aluminum nitride during a cooling step after soldering, which is performed at a temperature within the ramge of 500.degree. to 950.degree. C. This thermal stress may conceivably be left in the aluminum nitride substrate as tensile residual stress, to crack the aluminum nitride substrate and/or warp the radiating heat sink element of the copper-tungsten alloy or the copper-molybdenum alloy.
When an aluminum nitride substrate is joined to a radiating heat sink element of a copper-tungsten alloy or a copper-molybdenum alloy, by cold soldering or hot soldering, the aluminum nitride substrate or an interface-between the same and a metallized layer is cracked by a thermo cycle test (-55.degree. C. to +150.degree. C., 1000 cycles or a thermal shock test to cause a significant problem in practice, even if no warp nor crack is recognized at the time of joining.
In a sample of an aluminum nitride substrate joined to a radiating heat sink element of a copper-tungsten alloy or a copper-molybdenum alloy by silver soldering, thermal fatigue or thermal stress was caused in a thermo cycle test or a thermal shock test due to a difference between the thermal expansion coefficients of the radiating element of the copper-tungsten alloy or the copper-molybdenum alloy, and the aluminum nitride substrate, similarly to the above. Such a problem of thermal stress or thermal fatigue is aggravated with an increase in the junction area.
Thermal expansion coefficients of the copper-tungsten alloy or the copper-molybdenum alloy having the aforementioned composition and aluminum nitride are 6.5 to 10.times.10.sup.-6 /K and 4 to 5.times.10.sup.-6 /K respectively, within a range from room temperature to about 950.degree. C. Further, these materials, having a high Young's modulus of 27,000 to 35,000 Kg/mm.sup.2 and 35,000 to 37,000 Kg/mm.sup.2 respectively, are hardly plastically deformed. Thus, when the copper-tungsten alloy or the copper-molybdenum alloy of the aforementioned composition and aluminum nitride are joined with each other by soldering, a large thermal stress is conceivably caused in a cooling step.